Description of the problem, including code/CLI snippet

Adding multiple ProjectIssue instances from different projects into a set can result in missing elements.

Expected Behavior

The set should contain all unique issues.

Actual Behavior

Some issues are missing from the set.

Specifications

• python-gitlab version: 3.7
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): EE

iid has project scope, not group scope. It is used as the hashing key:

I think the hashing key should include more data from the issue. Perhaps just id will do. A similar problem exists for the ProjectIssue equality magic method.